I would say that the difference between libertarian and anarchist utopias is that the latter are more inclusive and libertarian ones more exclusive.

Anarchist utopias, although they can end up being restrictive too, at least tend to be based on the idea that eventually they will include the whole society, all the people. Everybody will be an anarchist living in an anarchistic society.

The libertarians tend to have visions of a supposedly libertarian society where in fact only a minority are libertarians and everybody else has to live based on their rules without being admitted to the ranks of the libertarians.

Just like in a bourgeois capitalistic society, the success of the social model demands that most people can't achieve the society's ideal. So, between leftwing and rightwing utopian visions we have the basic left/right divide.